The mechanisms responsible for the synthesis and packaging of peptide transmitters are distinct from those used for the small-molecule neurotransmitters but have similarities to the processes used for the synthesis of proteins that are secreted from nonneuronal cells (pancreatic enzymes).

Peptide neurotransmitters

Peptide-secreting neurons generally synthesize polypeptides that are much larger than the final, “mature” peptide. Processing these polypeptides, which are called pre-propeptides, takes place within the neuron’s cell body by a sequence of reactions that occur in several intracellular organelles. Pre-propeptides are synthesized in the rough endoplasmic reticulum, where the signal sequence—that is, the sequence of amino acids indicating that the peptide is to be secreted—is removed. The remaining polypeptide, referred to as the propeptide, moves through the Golgi apparatus and is packaged into vesicles in the trans-Golgi network. The final stages of peptide neurotransmitter processing involve proteolytic cleavage and occur within the Golgi-associated vesicles. In addition to cleavage, modification of the ends of the peptide by glycosylation, phosphorylation, and disulfide bond formation is also common.

Neuropeptides often are coreleased with small-molecule neurotransmitters. Thus, peptidergic synapses often elicit complex postsynaptic responses. Following release, these neuropeptides are inactivated into amino acid fragments by enzymes called peptidases that are typically located on the extracellular surface of the plasma membrane.

The biological activity of the peptide neurotransmitters depends on their amino acid sequence. Based on their sequences, neuropeptide transmitters have been loosely grouped into five categories:

1. Brain/gut peptides,
2. Opioid peptides,
3. Pituitary peptides,
4. Hypothalamic releasing hormones, and
5. Catch-all category containing “other.”

Substance P is an eleven-amino-acid peptide classified as a brain/gut peptide due to its expression in both the human hippocampus and neocortex and gastrointestinal tract. It is also released from C fibers—afferents in peripheral nerves that convey information about pain and temperature.

Substance P is a sensory neurotransmitter in the spinal cord, where its release can be inhibited by opioid peptides released from spinal cord interneurons, resulting in the suppression of pain.

Opioid peptides were initially discovered in the 1970s while screening for endorphins, or endogenous compounds that mimicked the actions of morphine. The endogenous ligands of the opioid receptors have been identified and fall into three classes: endorphins, enkephalins, and dynorphins.

Each ligand is synthesized in an inactive pre-propeptide derived from distinct genes. Opioid precursor processing is tissue-specific and carried out within the Golgi apparatus.

Opioid peptides are often colocalized with neurotransmitters, such as GABA and 5-HT, and are found widely distributed in many regions of the brain. In general, opioids tend to be depressants. Opioids are also involved in more complex behaviors such as sexual attraction and aggressive and submissive behaviors. They have also been implicated in psychiatric disorders such as schizophrenia and autism, although the evidence for this is debated. Repeated administration of opioids can lead to tolerance, addiction, or dependence; therefore therapeutic use is highly regulated.  There are three well-characterized opioid receptor subtypes (μ, δ, and κ) that play a role in reward mechanisms as well as addiction.

Neuropeptide receptors

Virtually all neuropeptides initiate their effects by activating G-protein–coupled receptors, however, studying these metabotropic peptide receptors in the brain has been difficult as there are few known specific agonists and antagonists. Neuropeptide receptor activation is important for a variety of homeostatic responses such as regulating the postganglionic output from sympathetic ganglia and the activity of the gut. Other peptide receptors, such as the neuropeptide Y receptor, are also involved in the initiation and maintenance of feeding behaviors that can lead to satiety or obesity, depending on regulation. Additional behaviors attributed to activation of peptide receptors include anxiety and panic attacks. These responses can be addressed through the use of antagonists of cholecystokinin receptors.

Unconventional neurotransmitters

There are also some unusual molecules that are used for signaling between neurons and their targets. These chemical signals can be considered as neurotransmitters because of their roles in interneuronal signaling and because their release from neurons is regulated by Ca²⁺. However, they are unconventional in comparison with other neurotransmitters because:

1. They are not stored in synaptic vesicles.
2. They are not released from presynaptic terminals via exocytotic mechanisms.
3. They may not be released from presynaptic terminals at all and are often associated with retrograde signaling (that is, from postsynaptic cells back to presynaptic terminals).

Endocannabinoids are a family of related endogenous signals that interact with cannabinoid receptors. These receptors are the molecular targets of Δ9-tetrahydrocannabinol, the psychoactive component of the marijuana plant.

Production of endocannabinoids is stimulated by:

1. A second messenger within postsynaptic neurons, typically a rise in postsynaptic Ca²⁺ concentration.
2. This allows these hydrophobic signals to diffuse through the postsynaptic membrane to reach cannabinoid receptors on other nearby cells.
3. Endocannabinoid action is terminated by carrier-mediated transport of these signals back into the postsynaptic neuron, where they are hydrolyzed by the enzyme fatty acid hydrolase (FAAH).

At least two types of cannabinoid receptors have been identified, with most actions of endocannabinoids in the CNS. The CB1 receptor is a G-protein–coupled receptor related to the metabotropic receptors for ACh, glutamate, and other conventional neurotransmitters.

Nitric oxide (NO) is an unusual and especially interesting chemical signal. It is a gas produced by the action of nitric oxide synthase, an enzyme that converts the amino acid arginine into a metabolite (citrulline) and simultaneously generates NO.

Within neurons, NO synthase is regulated by Ca²⁺ binding to the Ca²⁺ sensor protein calmodulin. Once produced, NO can permeate the plasma membrane and act inside nearby cells. Thus, this gaseous signal has a range of influence that extends well beyond the cell of origin. This property makes NO a potentially useful agent for coordinating the activities of multiple cells in a localized region and may mediate certain forms of synaptic plasticity that spread within small networks of neurons.

All of the known actions of NO are mediated within its cellular targets; for this reason, NO often is considered a second messenger rather than a neurotransmitter. Some of the actions of NO are due to the activation of the enzyme guanylyl cyclase, which then produces the second messenger cGMP within target cells.  Due to the gaseous nature of NO, termination of signal is difficult to characterize; NO will diffuse and the reduction in concentration is a likely termination.

Carbon monoxide (CO) is generated in neurons by the enzymatic cleavage of heme by heme oxygenase-2. CO binds hemoglobin and binds enzymes within the electron transport chain, however, in the case of the neuron, it is part of the neurovascular coupling mechanism and increases local vasodilation.

ATP and other purines

All synaptic vesicles contain ATP, which is coreleased with one or more “classical” neurotransmitters. It has been known since the 1920s that the extracellular application of ATP (or its breakdown products AMP and adenosine) can elicit electrical responses in neurons.

ATP acts as an excitatory neurotransmitter in motor neurons of the spinal cord, as well as in sensory and autonomic ganglia. Extracellular enzymes degrade released ATP to adenosine, which has its own set of signaling actions. Thus, adenosine cannot be considered a classical neurotransmitter because it is not stored in synaptic vesicles or released in a Ca²⁺-dependent manner. Several enzymes are involved in the rapid catabolism and removal of purines from extracellular locations. Receptors for both ATP and adenosine are widely distributed in the nervous system as well as in many other tissues. There are three classes of these purinergic receptors.  One class consists of ionotropic receptors called P2X receptors. The structure of these receptors is unique among ionotropic receptors because each subunit has a transmembrane domain that crosses the membrane only twice.  The other two classes of purinergic receptors, P2Y, are G-protein–coupled metabotropic receptors. The two classes differ in their sensitivity to agonists—one type is preferentially stimulated by adenosine, whereas the other is preferentially activated by ATP.